In the biology series, we discussed phylogeny and the tree of life.
In doing so, we outlined the manner in which we classify living organisms, which we refer
to as taxonomy.
If you haven’t seen that tutorial yet, I highly recommend giving it a watch before
moving forward, as it describes this very important classification system in detail,
as well as other relevant information about how the tree of life is structured.
But if you remember your basic taxonomy, let’s just quickly recall the various taxa.
In order of increasing specificity, these are domain, kingdom, phylum, class, order,
family, genus, and species.
Every living organism belongs to a particular species, many species can be of the same genus,
many genera can be of the same family, and so forth.
So to begin broadly, there are three domains of life, those being bacteria, archaea, and
eukarya.
The first two contain only unicellular prokaryotes, so head over to the microbiology series for
more on those critters.
Eukarya contains all the eukaryotic organisms, so that’s what we are concerned with here.
Now in 1866, it was proposed that there were four eukaryotic kingdoms.
These were Animalia, Plantae, Fungi, and Protista.
Though this system is still occasionally taught, it has been considered inaccurate for quite
some time.
This is largely due to the consistently confounding “protista” that actually do not represent
a unified kingdom.
The term “protist” is polyphyletic, and should be avoided.
It is essentially a wastebasket term, meaning it was defined by what it didn’t include
rather than what it did.
That is, if an organism was a eukaryote, but it wasn’t a plant, animal, or fungus, it
was classified as a “protist.”
With this misconception rectified, we can simply acknowledge these three main kingdoms.
Fungi are covered in the mycology series, and plants are covered in the botany series.
But animals are the focus of this zoology series, so kingdom animalia is where we will
spend all of our time.
Within animalia there are many different phyla, and that’s where things start to get very,
very complicated.
There are so many types of animals, and the criteria that we use to categorize them begin
to get blurry very quickly.
Nevertheless, we must do our best to press forward, so let’s talk about how we can
use phylogeny to guide our exploration of the kingdom Animalia, which allows us to arrive
at a cladogram that looks like this.
It’s important to emphasize that this is a hypothesis, a scientific proposal based
on the analysis of data, so it isn’t set in stone.
It could change in the future.
Think of it as a general guide to animal diversity, a roadmap for our exploration.
However, regardless of one’s immediate perception, it is designed to include all known animal
species.
That is, all extant animals can be categorized into one of the animal phyla that appear here.
So where did this cladogram come from?
It was first published by Drs. Gonzalo Giribet and Gregory D. Edgecombe, in their book The
Invertebrate Tree of Life, which is linked in the description below should you wish to
extend your learning beyond these tutorials.
It is important to note that Drs. Giribet and Edgecombe didn’t just make up this classification
on their own.
The creation of this tree was a massive undertaking.
It is the synthesis of a multitude of works, and as of today at least, represents our most
current understanding of animal phylogeny in the world.
This figure has been altered slightly from the original, but in a way that does not change
the phylogeny.
We have merely switched the placement of Ctenophora and Porifera and rearranged it somewhat so
that it more closely follows many zoology courses, meaning that we end with the chordates
instead of Brachiozoa, terms that will soon be understood as we work our way through the
series.
Here’s a look at the original figure compared to ours.
Notice that the two cladograms convey the same information, that is, their phylogeny
is the same, the only difference is the apparent “order” of organisms.
Something that must be mentioned is that this tree looks different from others that are
proposed.
This is due, in part, because the total number of animal phyla is still in flux.
In some modern cladograms there are only 29 phyla, while in others there are up to 36.
This is due to a few cryptic animals that make placement difficult, as well as recent
analyses that either break down defunct phyla, like Mesozoa, which is now split into Dicyemida
and Orthonectida, or reclassify phyla, like Pentastomida, which is now considered to be
part of Arthropoda.
Now, let’s take a closer look at this massive cladogram, which will be our guide as we move
through the series.
All of this daunting nomenclature will become clear over several dozen tutorials, but for
now let’s just try to take in as much as we can in order to get some important context.
Zoom in on this corner here and you’ll see the phyla Porifera and Ctenophora branch off
at the same time.
This is known as a polytomy because the organism represented by this internal node has more
than two immediate descendants.
Essentially, it means we’re not really sure which of these phyla are more “ancestral”
to the rest of the animal kingdom.
It has long been widely accepted that the sponges, which are found in phylum Porifera,
were the most “basal” animals, but this is misleading.
Again, sponges aren’t truly “basal” since they’ve been evolving for more than
500 million years within their own clade.
In the same way that an orangutan is not a “basal” hominid compared to a human, as
humans, chimps, bonobos, gorillas, and orangutans are all hominids, sponges are not some kind
of “more basal” animal.
Orangutans are equally distantly related to humans, gorillas, chimps, and bonobos.
That is, Pongidae is a sister clade to Homininae.
So again, this does not mean orangutans are more primitive, ancestral, or basal.
Likewise, it’s better to think of sponges as a sister clade to the rest of kingdom animalia.
However, recent analyses have placed the comb jellies, found in phylum Ctenophora, in that
position.
It is unclear if the “Porifera-sister” or the “Ctenophora-sister” hypothesis
is correct, but many textbooks still place the poriferans as the most basal and the choanoflagellates
as the sister clade to kingdom animalia.
So, that’s what we’ve done here.
Branching off from here you’ll see that our next classification involves the sister
groups of placozoans, cnidarians, and “all other animals”, that is, those classified
as Bilateria.
It has been a general consensus for years that the cnidarians are likely the sister
clade to all bilaterally symmetrical animals, though that leaves out placozoans.
What we are trying to convey is that the base of the animal phylogenetic tree is still controversial.
It is possible the arrangement of Porifera, Ctenophora, Placozoa, and Cnidaria will change
in the future.
But for now this is what we will work with.
Continuing on, we will notice that all remaining phyla are part of Bilateria, animals that
demonstrate bilateral symmetry in embryological development.
However it is the case that some animals, like many echinoderms, which refers to starfish
and sea urchins, lack this symmetry as adults.
The next node that branches off is Xenacoelomorpha, which is considered to be a sister group to
the Nephrozoa, and includes the clades Acoelomorpha and Xenoturbellida.
The clade Nephrozoa contains almost all animal phyla and well over one million named extant
species.
Nephrozoa is then broken up into the deuterostomes and the protostomes.
In order to make sense of these major groups, we have to look at early development, where
the differentiation of the cellular layers is taking place.
Once an egg cell is fertilized by a sperm cell it becomes a zygote, and an indentation
called the blastopore forms on the embryo.
Animals like echinoderms and chordates are called deuterostomes, or “second mouths,”
because in these animals the indentation of the blastopore becomes the animal’s anus.
As the blastopore tunnels through the embryo of a deuterostome animal, it creates the rest
of the digestive tract, ending with the mouth.
These will be the last animals covered in this series, and they include some of the
best-known animals, from sharks and stingrays to cats and primates.
In other animals, like segmented worms, mollusks, and arthropods, the initial opening in the
blastopore generally develops into the mouth.
These animals are called protostomes, or “first mouths.”
As the blastopore tunnels through the embryo of a protostome animal, it creates the rest
of the digestive tract, ending with the anus.
These protostome animals can be further divided into two main groups, the Spiralia and the
Ecdysozoa.
The group Ecdysozoa is monophyletic.
All Ecdysozoans possess an outer covering called the “cuticle” that is periodically
shed.
This includes nematodes, arthropods, tardigrades, velvet worms, and others.
Spiralia meanwhile, includes the mollusks, annelids, platyhelminths and many others.
The term “Spiralia” refers to spiral cleavage, a pattern of early development found only
in members of this clade.
Spiralia includes an incredible diversity of animal phyla and is notoriously difficult
to define based on morphology.
This clade in particular has undergone many dramatic revisions compared to Ecdysozoa and
Deuterostomia.
It is likely there will be further clarifications of the Spiralia clade, but its position as
a sister-group to Ecdysozoa is unlikely to change.
Now, as we’ve been going over this cladogram you’ve probably noticed a few phyla you’re
familiar with as well as quite a few that you’ve never heard of.
The animal phyla that contain the vast majority of all animal life, sometimes referred to
as the “nine animal phyla” are Arthropoda, which includes the insects, arachnids, and
crustaceans, Mollusca, which includes the snails, slugs, cephalopods, and bivalves,
Chordata, which includes all animals with a backbone, like snakes, birds, humans, and
whales, Platyhelminthes, which includes the free-living and parasitic flatworms, Nematoda,
which includes the roundworms, Annelida, which includes mostly segmented worms, like the
earthworms, Cnidaria, which includes the corals and jellyfish, Echinodermata, with includes
the sea stars and sea cucumbers, and Porifera which includes the sponges.
However, as you can see, there are many other animal clades and, in this series, we are
going to be discussing all of them.
Now, let’s not get too far ahead of ourselves.
Before we dive head-first into this astounding web of animal diversity, we need to cover
the formation of kingdom Animalia, so let’s move forward and get just a bit more evolutionary
context.